ECLIPSE Compositional Simulator: The Asphaltene Chuck Kossack Schlumberger Advisor Denver, Colorado 1 NTNU Lecture Brief overview of Asphaltene in ECLIPSE Compositional Simulator Look at theory skip keywords 2
Asphaltene in oil can Precipitate Deposit Introduction in the reservoir, wellbore, tubing, and transport lines. Asphaltene, once deposited, can: Damage the formation Change the wettability of the rock surface Block perforations Clog pipelines It is important to understand the processes by simulating them. 3 Introduction Paraffins, asphaltenes, and resins are the typical sources of organic deposition in wells, pipelines, and reservoir formation, during petroleum production. Organic deposition can occur both on the surfaces of well tubing and formation pores to reduce the flow efficiency and eventually to clog the flow paths completely. 4
Introduction Asphaltene precipitation and deposition causes: Changes in Pressure, Temperature, Composition Shear rate 5 Asphaltenes Term The term originated in 1837 J.B. Boussingault defined asphaltenes as Residue of the distillation of bitumen: Insoluble in alcohol and soluble in turpentine 6
Current Definition Asphaltenes are insoluble in n-alkanes such as n-pentane n-heptane Soluble in toluene 7 Current Definition Asphaltenes are high molecular weight aromatic organic substances soluble in aromatic solvents (e.g., toluene, diesel) but precipitated by the addition of molecular-weight alkenes (e.g. n- heptane/n-pentane) 8
Physical Properties Asphaltenes dark colored friable solids density of about 1.2 g/cm3 Infusible, meaning have no defined melting point, but decompose when heated, leaving a carbonaceous residue 9 Physical Properties Molecular weight of asphaltene ranges from 1000 to several hundreds of thousands Asphaltenes tend to remain in solution or colloidal suspension under reservoir temperature and pressure conditions 10
Asphaltene Solubility Asphaltene solubility is highly dependent on the composition of the crude High asphaltene content of crude does not necessarily mean that flowing problems will occur during production Often the contrary is the case examples next slide 11 Asphaltene Solubility Examples Venezuelan Boscan crude, which is very heavy, containing 17% of asphaltenes, was produced nearly trouble free. Algerian Hassi-Messaond crude, which is very light, contains only 0.062% of asphaltenes, met with difficulties during production. 12
Three States of Asphaltene Asphaltene is dissolved in oil. When its concentration is above its solubility, it precipitates to form fine particles precipitation. Fine particles can form larger particles (flocs) flocculation. Flocs can also be dissociated into fine particles. 13 States of Flocs Flowing with oil Adsorbed onto the surface of the rock adsorption Plugged at the pore throat plugging Flushed away with oil entrainment (high velocity flow) 14
Asphaltene Precipitation Start to precipitate once the stability of the colloidal suspension is destabilized Caused by the changes in temperature or pressure during primary depletion Also asphaltenes become unstable as a result of fluid blending (comingling) of fluid streams as well as by gas injection during improved oil recovery (IOR) operations 15 Typical Asphaltene Molecules Asphaltene molecular structures. Shown here are three of the many possible structures of asphaltenes, which are a class of molecules composed of grouped aromatic rings (blue) with alkane chains. Some rings may be nonaromatic. Many of the rings are fused, meaning the rings share at least one side. 16
Asphaltene Phase Behavior The highest pressure at which asphaltenes are first for a given oil or oil/solvent mixture is referred to as the upper onset pressure. As pressure is reduced below the onset pressure, further precipitation of asphaltenes occurs, and asphaltene particles flocculate into larger aggregates. 17 Asphaltene-Precipitation Envelope 18
Asphaltene Precipitation Asphaltenes precipitated from the oil - may continue to flow as suspended particles Or may deposit onto the rock surface causing plugging Asphaltenes on rock may alter wettability Deposition begins with adsorption of flocculated asphaltene particles onto active sites on the rock surface Mainly on high specific area clay minerals such as kaolinite 19 Asphaltene Precipitation Next have hydrodynamic retention or trapping of particles at the pore throats Deposition of solid asphaltenes causes a reduction of the pore space (porosity) available for fluids Other formation damage mechanisms may include permeability reduction alteration of rock wettability from water wet to oil wet 20
Asphaltene Re-dissolving/Re-entraining Deposited asphaltene particles also may be re-entrained in the flowing oil stream due to a mechanical erosion or ablation effect if the interstitial velocity of the fluids becomes high enough Re-entrained means asphaltene particles are still solid but now flow with fluid 21 Changes Due to Asphaltene Deposition Reduced pore volume Decreased absolute permeability Decreased oil viscosity Increased due to precipitation Changed wettability More oil-wet Increased SOWCR, KRW Decreased SWCR, KROW Less oil-wet More oil-wet 22
End of Theoretical Discussion Several key papers and reports are provided with course data sets Many SPE papers available on this subject We will now look at ECLIPSE Compositional treatment 23 ECLIPSE E300 The Asphaltene 24
Overview of ECLIPSE Treatment Asphaltene phase behavior, precipitation, effect on fluid flow, etc Very complex Not understood well Some disagreement between experts on how to handle it 25 ECLIPSE Technical Description Disclaimer The purpose of the Asphaltene option is to model asphaltene precipitation and its impacts on reservoir performance. There are a number of asphaltene models currently in existence, but there is still no consensus about the characterization of asphaltene behavior. 26
Definitions Precipitation: the process that triggers the appearance of the asphaltene particles ( fines ). Flocculation: the process whereby the precipitated fines aggregate to form bigger particles, ( flocs ). Deposition: the mechanistic process of exchanging flocs between the liquid solution (oil) and the solid matrix making up the reservoir rock. 27 Asphaltene Modeling Decomposed into Different Stages The double arrow indicates reversibility (partial or total). --------------------------------------------------------- Precipitation triggers the sequence of 1 st - flocculation 2 nd - deposition leading to damage And including a viscosity effects 28
Components in Data Set CNAMES C1 C3 C6 C10 C15 C20 ASPH / ZMFVD -- 1 2 3 4 5 6 7 -- P C1 C3 C6 C10 C15 C20 ASPH 1000.0 0.50 0.03 0.07 0.20 0.15 0.05 0.00 10000.0 0.50 0.03 0.07 0.20 0.15 0.05 0.00 / ASPH component will be the flocs precipated asphaltene that is mobile C20 is the disolved asphaltene 29 Precipitation 30
Effect on Composition During Precipitation Note that the precipitation process does not change the fluid composition. It only flags the fraction of a precipitate component that will contribute to the flocculation process. 31 Summary: Precipitation Asphaltene is assumed to be dissolved into oil. Fine particles appear when its weight percentage in oil is above a limit. This limit can be a function of pressure. Re-dissolution Precipitation Remaining Asphaltene after precipitation (WT%) Lower onset pressure Bubble point pressure Upper onset pressure 32 Pressure
Asphaltene Flocculation-Dissociation l i i 33 Asphaltene Flocculation-Dissociation Fines Flocs Flocculation Dissociation 34
Asphaltene Flocculation-Dissociation Flocculation is the process whereby fines obtained from the precipitation process aggregate to form bigger particles referred to as flocs. This process is modeled by a set of two kinetic reactions to allow reversibility (partial or total) between aggregation of the fines into flocs and dissociation of the flocs into fines. 35 Flocculation and Dissociation R a = aggregation rate of the fines i into flocs a R a = C C t a = r ia C i r ai C a Precipitations (fines) of components i, i+1, j form flocs represented by component a C i concentration of fines from component I C a concentration of flocs r ia flocculation rate coefficient r ai dissociation rate coefficient 36
Asphaltene Deposition 37 Asphaltene Deposition Once the asphaltene component become flocs they are available to deposit or adsorb onto the rock OR become trapped in the rock pores plugging the porous medium The adsorbed asphaltene component can desorbs or entrain back into the liquid phase when the velocity (shear rate) is high enough. 38
Asphaltene Entrainment (Opposite of Deposition) The flocs can be entrained and returned to the oil phase because of high, local velocity (shear). 39 Deposition Three components: Adsorption Plugging Entrainment Rate of deposition = Rate of Adsorption + Rate of plugging Rate of Entrainment 40
Rate of Adsorption Adsorption rate = α φ C a where: α adsorption coefficient φ - porosity C a volumetric concentration ti of the flowing flocs 41 Rate of Plugging Plugging rate = γ F o C a Where F o oil Darcy flux C a volumetric concentration of flowing flocs γ plugging coefficient 42
Rate of Entrainment Rate of entrainment = β ( U o -U cr )ε if U o >U cr = 0 if U o <=U cr Where: U o - oil flow velocity in the pore U cr - critical oil velocity β ε - entrainment rate - deposition fraction 43 Asphaltene Deposition Putting It All Together We get the following equation: 44
Asphaltene Deposition The deposition in the flow direction i is modeled as follows ε i α + = φ Ca + γ Foi Ca β ( U oi U cr ) t d Where d is the dimension of the problem (1,2 or 3) ε i is the volume fraction of deposit in the i direction of the flow α is the adsorption or static deposition coefficient φ is the current porosity (time t) C a F oi γ is the volumetric concentration of the flocs in the oil phase (flowing flocs) is the oil Darcy flux is the plugging coefficient β is the entrainment coefficient 45 ε i Damage 46
Reduction in Porosity φ = φ φ 0 t ε dt t 0 φ 0 is the initial porosity, ε is the volume fraction of asphaltene deposit 47 Reduction in Absolute Permeability As a function of porosity K K 0 ε 1 φ = 0 δ Defined in a table δ is defined in ASPKDAM ASPKDAM --ε k/k 0 1e-5 0.99 2e-5 0.98 5e-5 0.95 1e-4 0.90 2e-4 0.88 / 48
Increase in Viscosity Generalized Einstein model Item 3 of ASPHALTE = EI = 1 + ac μ 0 a is defined in ASPVISO Krieger and Dougherty model Item 3 of ASPHALTE = KD h and C po are defined in ASPVISO Cp - volume fraction of precipitates Look up table Item 3 of ASPHALTE = Viscosity multipliers vs. volume fraction of precipitates in ASPVISO μ ASPVISO --C p μ / μ 0 0.0 1.0 0.01 1.2 0.1 1.5 1.0 5.0 / 1 + ac p μ C = 1 μ 0 C p p0 η C p 0 49 Change in Wettability 50
Change in Wettability Rock can change from water wet to more oil wet when asphaltene deposits. No wettability change if Pcow is 0 or negative. Kr is between Kr_w-wet and Kr_o-wet. oil-wet water-wet 51 End of Lecture (Data set with all details and Keywords is available) 52